27. SMT terminology introduction

1. Template: (steel mesh)

First, determine whether to process the template according to the designed PCB. If the chip components on the PCB are only resistors and capacitors and the package is more than 1206, the template can be used without a template, and the solder paste can be applied with a syringe or an automatic dispensing device; when the PCB contains SOT, SOP, PQFP, PLCC and The chip of the BGA package and the package of the resistor and capacitor must be made of a template of 0805 or less. The general template is divided into chemically etched copper template (low price, suitable for small batch, test and chip lead pitch 0.635mm); laser etched stainless steel template (high precision, high price, suitable for high volume, automatic production line and chip lead pitch 0.5mm). For R&D, small batch production or 0.5mm pitch, etched stainless steel stencils are recommended; for mass production or laser cutting of stainless steel stencils with a pitch of 0.5mm. The outer dimensions are 370*470 (unit: mm), and the effective area is 300*400 (unit: mm).

2. Silk screen: (high-precision semi-automatic solder paste printing machine)

Its function is to use a scraper to print solder paste or patch glue onto the pads of the PCB to prepare the components for mounting. The equipment used is a manual screen printing station (screen printer), a stencil and a scraper (metal or rubber) at the forefront of the SMT line. It is recommended to use a medium screen printing station, a precision semi-automatic screen printing machine method to fix the template on the screen printing table, determine the position of the PCB on the screen printing platform through the upper and lower and left and right knobs on the manual screen printing table, and fix the position; then apply the desired Place the PCB between the screen printing platform and the template, place solder paste on the screen plate (at room temperature), keep the template parallel to the PCB, and apply the solder paste evenly on the PCB with a doctor blade. During the use, pay attention to the timely cleaning of the template with alcohol to prevent the solder paste from clogging the leak of the template.

3. Mounting: (Korea high-precision automatic multi-function placement machine

Its role is to accurately mount surface mount components to a fixed location on the PCB. The equipment used is a placement machine (automatic, semi-automatic or manual), vacuum pen or tweezers, located behind the screen printing station in the SMT production line. For the laboratory or small batches, it is recommended to use a double-head anti-static vacuum pen. In order to solve the placement and alignment problems of high-precision chips (chip pitch 0.5mm), it is recommended to use South Korea's Samsung automatic multi-function high-precision placement machine (model SM421 can improve efficiency and placement accuracy). The vacuum pen can directly pick up the resistor, capacitor and chip from the component rack. Since the solder paste has a certain viscosity, the resistor and capacitor can be placed directly at the desired position; for the chip, a suction cup can be added to the vacuum pen tip. The amount of suction can be adjusted with the knob. Remember to pay attention to the alignment position regardless of the components placed. If the position is misaligned, you must clean the PCB with alcohol, re-screen, and reposition the components.

4, reflow soldering:

Its function is to melt the solder paste, so that the surface mount components and the PCB are brazed together to achieve the electrical performance required by the design and are precisely controlled according to the international standard curve, which can effectively prevent thermal damage and deformation of the PCB and components. . The equipment used is a reflow oven (automatic infrared hot air reflow oven) located behind the placement machine in the SMT production line.

5, cleaning:

Its function is to remove the electrical properties of the mounted PCB or solder residues such as flux, etc., if the use of no-clean solder can generally not be cleaned. For products that require micro-power products or high-frequency characteristics, they should be cleaned, and general products can be cleaned free. The equipment used is an ultrasonic cleaner or directly washed with alcohol, and the position may not be fixed.

6, inspection:

Its role is to test the quality and assembly quality of the mounted PCB. The equipment used has a magnifying glass, a microscope, and the position can be placed in a suitable place on the production line according to the needs of the inspection.

7, repair:

Its role is to rework PCBs that detect faults, such as solder balls, tin bridges, open circuits and other defects. The tools used are smart soldering irons, rework stations, etc. Configure anywhere in the production line.

four. SMT assisted process: mainly used to solve the wave soldering and reflow soldering mixing process.

1, printing red plastic: (can also print red plastic)

The function is to print the red plastic to the fixed position of the PCB. The main function is to fix the components to the PCB. Generally, it is used for surface mount components on both sides of the PCB and one side for wave soldering. The equipment used for printing press paste and red offset printing can be completed by one machine and is located at the forefront of the SMT production line.

2, curing: (reflow soldering for curing and lead solder paste effect is better)

Its function is to cure the patch adhesive, so that the surface mount components and the PCB are firmly bonded together. The equipment used is a curing oven (reflow ovens can also be used for the curing of glues and heat aging tests for components and PCBs), located behind the placement machine in the SMT line.

Conclusion:

SMT surface mount technology includes many aspects, such as electronic components, integrated circuit design and manufacturing technology, electronic product circuit design technology, automatic placement equipment design and manufacturing technology, auxiliary materials development and production technology used in assembly manufacturing, electronics Product anti-static technology, etc., therefore, the production of a complete, beautiful, system test performance of electronic products will have many factors.

SMT full process